Theory one: Cold air and inflammation

When we ask our bodies to do ridiculously tough exercise – like running a 10km completely untrained or climbing 1504 steps as quick as you can – we switch from breathing through our nose to solely breathing through our mouth, to get as much oxygen in our lungs as possible.

As Mitchell explains, when we breathe through our nose, the air is actually heated up on the way in, which is far easier for your lungs to tolerate.

But when we breathe through our mouth, we get a huge rush of potentially cold air entering the lungs, which could cause some irritation. This effect gets particularly worse if you're in a cold, dry environment like the stairwell of a building or a high-altitude mountain.

"Cold, dry air can irritate the delicate mucosal linings of the airways and cause inflammation — which explains why sometimes we can feel tight chested after exercise in cold weather — or even cause minor tears in the lining," says Mitchell.

"This then allows small amounts of blood to enter the airways."

Theory two: Air pressure

If literally swallowing your own blood sound dubious to you, there's also a second theory which offers an explanation for why we feel like vampires during really tough bouts of exercise.

"The second theory is that when we undertake high-intensity work, especially seen in traditional HIIT protocols above 95 percent maximal heart rate, we undergo immense physiological stress," says Mitchell.

"The air pressures exerted on the lungs, in particular the air sacs (alveoli), during these sorts of intensities can allow red blood cells to enter into the alveoli."

In simple terms, when you push your body to the limits during intense exercise the air pressure in your lungs is so great that the blood cells surrounding the little pockets of air in your lungs can burst, releasing tiny droplets of blood into your airways.

(Just try using that as an excuse next time your PT makes you do burpees.)

Is it something you should worry about?

While it’s a pretty dumb idea to push yourself to breaking point every time you work out, tasting blood during a HIIT session every once in a while is generally nothing you should be worried about.

"If you’re generally in good health, regularly physically active and it isn’t occurring continually then no, you shouldn’t be concerned about it," says Mitchell.

"However, if it becomes a regular thing then it may indicate a much more sinister underlying cause that should be investigated by a medical professional."